host-resolver: Do not cancel threads waiting for new queries during cleanup

host-resolver: Do not cancel threads waiting for new queries during cleanup

While it is currently unclear why it happens, canceling threads waiting in the
new_query condvar does not work as expected. The behavior is not fully
reproducible: Either cancel(), join() or destroying the condvar hangs.

The issue has been seen in the http-fetcher unit tests, where the stream service
triggers the use of the resolver for "localhost" hosts. It is reproducible with
any cleanup following a host_create_from_dns() use on a Ubuntu 14.04 x64 system.
Further, the issue is related to the use of libunwind, as only builds with
--enable-unwind-backtraces are affected.

As we broadcast() the new_query condvar before destruction, a hard cancel() of
these threads is actually not required. Instead we let these threads clean up
themselves after receiving the condvar signal.